This invention relates to geohydrology studies, and more particularly to a system by which to obtain a multitude of groundwater samples at a multitude of precise elevations below ground level for selective chemical analyses, water temperature and pressure measurements, in a minimum of time and at a minimum of cost.
Groundwater sampling systems provided heretofore generally utilize individual or nested groups of one to seven rigid steel or plastic pipes of 2.5 to 5 centimeters inside diameter, each with a bottom pipe section perforated over a length of 1 to 12 meters, and obtaining groundwater samples therefrom by the manual and time consuming use of bailers, or powered pumps inserted deeply into the pipe. The rigid pipes are installed in the ground by the provision of drill holes of desired depth, and the bottom perforated section of each pipe is terminated at a different depth.
Because of drill hole entry size limitations and escalating costs with increasing drill hole size, most nested groups of sampling wells are formed of individual, closely adjacent drill holes of about 15 to 25 centimeters in diameter, drilled to different depths, with each containing a single 5 centimeter diameter perforated pipe. Occasionally, a single 15 centimeter drill hole is provided with two or three perforated rigid pipes of about 3 centimeters in diameter. More rarely, a 25 centimeter diameter drill hole may be provided with up to seven 2.5 centimeter perforated pipes.
Prior art groundwater sampling systems are disclosed in Construction Dewatering--A Guide To Theory And Practice, by J. Patrick Powers, P. E., Published 1981 by John Wiley and Sons, Inc., pages 133-136 and 271-275, and in Groundwater Monitoring Review, Spring 1986, Volume 6, No. 2, Published by Water Well Journal Publishing Company, generally through pages 50-72 and many advertisements throughout the Volume, and in particular the article on pages 50-55 entitled Multiple Completion Monitor Wells, by Dennis B. Nakamoto et al.
The foregoing systems of the prior art have many serious limitations and disadvantages: They require the removal of large volumes (4 to 10 pipe volumes) of stagnant and usually contaminated fluid prior to groundwater measurement in order to insure representative, fresh chemical sampling of the porous medium. Cross-contamination from pipe-to-pipe is common because bailers and pumping equipment are difficult to clean after each use. The perforated sections of pipe are subject to irreversible clogging of the rigid perforations by earth formation particles and naturally precipitated mineral cements. They are more subject to leakage because of the multiplicity of pipe joints and metal corrosion, and to breakage during installation or long term earth movements. They do not provide sufficient precision in mapping chemical and pressure variations in a porous medium, at reasonable cost. And they do not allow simultaneous and rapid measurements of fluid pressure change during formation pump tests without sophisticated recording devices or extra manpower.
Structurally complex large diameter ported rigid pipe assemblies also are available at very high cost. Examples of components thereof are disclosed in U.S. Pat. Nos. 4,192,181; 4,204,426; 4,230,180; 4,254,832; and 4,258,788. These assemblies are subject to many of the limitations and disadvantages of the conventional systems described hereinbefore.